Ring-closing metathesis (RCM) is used for synthesizing cyclic compounds.
RCM has played a key role in the generation of cyclic motifs since Grubbs and co-workers reported the now well-defined ruthenium catalysts. (See, for example, Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem. Int. Ed. Engl. 1995, 34, 2039.) Grubbs catalysts and Hoveyda-Grubbs catalysts have found wide application, including in the synthesis of various heterocyclic and macrocyclic natural products, and other polymers. However, industry, including the pharmaceutical industry has not yet widely adopted RCM in large-scale manufacturing. Reasons for this include the high cost of the ruthenium-containing compounds, and the relatively high metal leaching.
Attempts have been made to immobilize these homogeneous catalytic complexes on several types of supports. Many groups have reported on the immobilization of the first- and second-generation Grubbs' catalysts (see, for example, Seiders, T. J.; Ward, D. W.; Grubbs, R. H. Org. Lett. 2001, 3, 3225). Others have reported on the immobilization of reusable modifications of these catalysts (see, for example, Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 791; and Gessler, S.; Randl, S.; Blechert, S. Tetrahedron Lett. 2000, 41, 9973). Immobilization techniques have been reported using: soluble polymers (see, for example, Yao, Q. Angew. Chem. Int. Ed. Engl. 2000, 39, 3896; and Yao, Q.; Motta, A. R. Tetrahedron Lett. 2004, 45, 2447); insoluble polymers (see, for example, Barrett, A. G. M.; Camp, S. M.; Roberts, R. S. Org. Lett. 1999, 1, 1083; Nieczypor, P.; Buchowicz, W.; Meester, W. J. N.; Rutjes, F. P. J. T.; Mol, J. C. Tetrahedron Lett. 2001, 42, 7103; Akiyama, R.; Kobayashi, S. Angew. Chem. Int. Ed. Engl. 2002, 41, 2602; Grela, K.; Tryznowski, M.; Bieniek, M. Tetrahedron Lett. 2002, 43, 9055; and Halbach, T. S.; Mix, S.; Fischer, D.; Maechling, S.; Krause, J. O.; Sievers, C.; Blechert, S.; Nuyken, O.; Buchmeiser, M. R. J. Org. Chem. 2005, 70, 4687); monolithic gels (see, for example, Kingsbury, J. S.; Garber, S. B.; Giftos, J. M.; Gray, B. L.; Okamoto, M. M.; Farrer, R. A.; Fourkas, J. T.; Hoveyda, A. H. Angew. Chem. Int. Ed. Engl. 2001, 40, 4251); ionic liquids (see, for example, Audic N.; Clavier, H.; Mauduit, M.; Guillemin, J.-C. J. Am. Chem. Soc. 2003, 125, 9248; and Yao, Q.; Zhang, Y. Angew. Chem. Int. Ed. Engl. 2003, 42, 3395); fluorous materials (see, for example, Yao, Q.; Zhang, Y. J. Am. Chem. Soc. 2004, 126, 74; and Matsugi, M.; Curran, D. P. J. Org. Chem. 2005, 70, 1636); and silica (see, for example, (Melis, K.; Vos, D. D.; Jacobs, P.; Verpoort, F. J. Mol. Catal. A: Chem. 2001, 169, 47). However, immobilized catalysts obtained by these methods generally also suffer from shortcomings, such as, low reactivity (e.g., due to diffusion-related issues), reduced activity upon reuse, requirement for further purification, etc.
Ring-opening metathesis polymerization (ROMP) by (a) glass-type (see, for example, Kingsbury, J. S.; Garber, S. B.; Giftos, J. M.; Gray, B. L.; Okamoto, M. M.; Farrer, R. A.; Fourkas, J. T.; Hoveyda, A. H. Angew. Chem. Int. Ed. Engl. 2001, 40, 4251) and (b) one-pot functionalized monolith (see, for example, Sinner, F.; Buchmeiser, M. R. Macromolecules 2000, 33, 5777; and Mayr, M.; Mayr, B.; Buchmeiser, M. R. Angew. Chem. Int. Ed. Engl. 2001, 40, 3839) have also been reported. Both seem to be complicated procedures, despite the apparent advantages of application to library generation and ability to be recycled.
A silica gel-supported metathesis catalyst has also been reported, with reported mild reaction conditions, high turnover number and ease of purification. (See Fischer, D.; Blechert, S. Adv. Synth. Catal. 2005, 347, 1329.) However, this catalyst does not recycle well, even for reaction involving a simple substrate. Only 68% yield was reported achieved in three runs.
Others have reported on the use of mesoporous compositions as supports for catalysts. (See, for example, U.S. Pat. No. 6,544,923 and Xiaohua Huang, Chem. Comm., 2007, DOI:10.1039/b615564).
Siliceous mesocellular foams (MCF) have been prepared having a three-dimensional, interconnected pore structure with ultralarge cell-like pores (e.g., 24-42 nm) that are connected by windows of, for example, 9-22 nm. (See, for example, Schmidt-Winkel, P.; Lukens, W. W., Jr.; Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1999, 121, 254; Schmidt-Winkel, P.; Lukens, W. W., Jr., Yang, P.; Margolese, D. I.; Lettow, J. S.; Ying, J. Y.; Stucky, G. D. Chem. Mater. 2000, 12, 686; Lettow, J. S.; Han, Y. J.; Schmidt-Winkel, P.; Yang, P.; Zhao, D.; Stucky, G. D.; Ying, J. Y. Langmuir 2000, 16, 8291; Lettow, J. S.; Lancaster, T. M.; Glinka, C. J.; Ying, J. Y. Langmuir 2005, 21, 5738; and Yu, H.; Ying, J. Y. Angew. Chem. Int. Ed. 2005, 44, 288.)
In general, known procedures have not generated sufficiently efficient heterogenized catalysts for industrial applications. These immobilized catalysts have generally been difficult to prepare with high cost, poor environmental compatibility, and relatively poor catalytic activity.